The invention relates to the field of integrated POTS/DSL line cards, and more specifically to suppression of POTS transients in integrated POTS/DSL line cards.
Traditional deployment of digital subscriber line (DSL) service in addition to existing plain old telephone service (POTS) consists of separate DSL and POTS line cards that are usually housed in separate systems. A line card is typically the first circuit card that a subscriber's twisted pair telephone line encounters when it interfaces with the telephone network at the central office (CO). It is the first point of access for either digital or analog communication over the twisted pair. In order to couple the two services onto the subscriber loop, a POTS splitter is required at both the central office and subscriber locations.
A POTS splitter serves two functions. A first and most obvious function is to separate the low frequency POTS band from a higher frequency DSL band and to present these distinct signals to the respective equipment. A second function is to match the respective POTS and DSL signals to the proper termination impedance. Referring to FIG. 1, a graph illustrates typical voiceband and databand bandwidths for ADSL. The voiceband ranges from 0 to 4 kHz and the databand ranges from 30 kHz to 1.1 MHz. Referring to FIG. 2, a graph illustrates the characteristics of POTS and ADSL (Asymmetric Digital Subscriber Line) loop impedance. As illustrated, the impedance varies from complex in the POTS band to resistive in the ADSL band.
Referring to FIG. 3, a traditional CO splitter placement is illustrated generally by numeral 200. The splitter 210 is connected to the loop 220 and includes a HPF (High-Pass Filter) 230 and a LPF (Low-Pass Filter) 240. An ADSL transceiver 250, including a base-band modem 260 and an AFE (Analog Front End) 270, is connected to the high-pass filter 230. A POTS interface 280, including a CODEC (COder-DECoder) 290 and a SLIC (Subscriber Line Interface Circuit) 295, is connected to the low-pass filter 240. In general, the splitter 210 separates the POTS and ADSL frequency bands so that a common loop may be used.
Disadvantages of this traditional configuration include the need for added equipment, increased physical space requirements, and the cost of the central office POTS splitters. Moreover, deployment of DSL service with existing POTS service typically requires a skilled craftsperson to install and connect the splitter, which results in additional cost to the service provider.
To overcome these problems, integrated POTS/DSL line cards have been proposed. A more complete description of an integrated POTS/DSL line card can be found in U.S. Pat. No. 6,295,343, entitled “Method and Apparatus for Combining Voice Line Card and xDSL Line Card Functions”, assigned to the assignee of the present application, and which is hereby incorporated by reference.
However, POTS signaling activities cause transients on the subscriber loop that can interfere with the DSL service that shares the loop. Typically, these transients consist of low frequency noise. POTS signaling activities and associated transients include ringing, ring trip, on-off/off-on hook, and dial pulse. In order to provide ring signaling to the subscriber POTS terminal, a 20 Hz (i.e., in North America), nominal 86 Vrms sine wave is applied to the subscriber loop. Ring trip occurs when the subscriber POTS terminal goes off hook during the time when the ring voltage is applied to the subscriber loop. The sudden change in subscriber terminal impedance as the terminal goes off hook results in a voltage transient at the line card receive interface. Since the subscriber can go off hook during any part of the ring cycle, it is possible to generate large amplitude low frequency transient signals especially when the subscriber set goes off hook at or near the peak of the ring voltage waveform. An on-off/off-on hook, low frequency transient is generated by a change in POTS terminal impedance when the subscriber lifts the handset or replaces it on the receiver. A dial pulse transient is generated by a series of timed hook switch closures used for digit collection at the line card. The transients produced are similar to the on-off hook transients but are periodic in nature.
One disadvantage of known integrated POTS/DSL line cards is that they do not suppress these transients adequately. A need therefore exists for an effective means of suppressing transients caused by POTS signaling activity for integrated POTS/DSL line cards.
Consequently, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.